Inequivalence of Pure State Ensembles for Open Quantum Systems: The Preferred Ensembles are Those That Are Physically Realizable

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An open quantum system in steady state ?-^ ss can be represented by a weighted ensemble of pure states ?-^ ss = SkPk|?k> <?k| in infinitely many ways. A physically realizable (PR) ensemble is one for which some continuous measurement of the environment will collapse the system into a pure state |?(t)>, stochastically evolving such that the proportion of time for which |?(t)> = |?k> equals Pk. Some, but not all, ensembles are PR. This constitutes the preferred ensemble fact. We present the necessary and sufficient conditions for a given ensemble to be PR, and illustrate the method by showing that the coherent state ensemble ...View more >An open quantum system in steady state ?-^ ss can be represented by a weighted ensemble of pure states ?-^ ss = SkPk|?k> <?k| in infinitely many ways. A physically realizable (PR) ensemble is one for which some continuous measurement of the environment will collapse the system into a pure state |?(t)>, stochastically evolving such that the proportion of time for which |?(t)> = |?k> equals Pk. Some, but not all, ensembles are PR. This constitutes the preferred ensemble fact. We present the necessary and sufficient conditions for a given ensemble to be PR, and illustrate the method by showing that the coherent state ensemble is not PR for an atom laser.View less >